The present invention relates to sensitometers and in particular to a dual color sensitometer having an improved electroluminescent lamp for uniformily illuminating a surface with either of two colors.
In the processing of film, particularly X-ray film, it is desirable, for quality control purposes, to use a device, known as a sensitometer, for exposing a piece of test film to a known amount of light of a known color. The film is typically exposed through a multiple-step density gradient known as a step tablet or step wedge. The exposed image of the graduated step tablet on the film is then measured with an optical densitometer which determines the density produced by the sensitometer exposure on the various steps of the step tablet. Through such densitometer measurements, the speed, contrast, and "base plus fog" of the film can be determined. As used herein, "base plus fog" refers to step "zero"; i.e., the area just past the least-exposed step on the sensitometer strip. Its density is the inherent density of the film base plus any additional density resulting from film age, natural radiation exposure, etc.
Densitometer measurements of sensitometer exposures are commonly used in quality control of routine photographic processes, such as the use of automatic processors for the processing of X-ray film. The measurements are typically made each day and plotted to assess daily and weekly processor variations to determine if a processor or film problem is present.
In general, there are two different kinds of photographic film used in X-ray and other applications. One is sensitive mainly to blue light and the other is also sensitive to green light. The most desirable wavelength for exposing blue film is 450 nanometers while the most desirable wavelength for exposing green sensitive film is 550 nanometers. Since both blue and green light sensitive film are frequently used, it is desirable to provide a sensitometer that can selectively emit light in either the blue or green region.
A common type of light source for use in a sensitometer is an electroluminescent lamp. An electroluminescent lamp is a light source consisting of a phosphor disposed between two conductive electrodes, with one electrode being transparent. When an AC voltage is applied between the electrodes the phosphor emits light. While the wavelength at which most electroluminescent phosphors emit light is independent of the drive frequency of the AC signal, phosphors have been developed which exhibit a distinct color shift with variations in the frequency of the signal applied to the lamp. However, such color shifts are relatively small in magnitude. For example, a typical lamp which emits light at a wavelength of 480 nm when operated at 60 Hz. may emit light at a wavelength of 505 nm when operated at 400 Hz. Prior art dual color sensitometers have been developed which employ electroluminescent phosphors that operate under this principle. The disadvantage with this approach, however, is that it is difficult to achieve the desired degree of shift in wavelength appropriate for blue and green sensitive films. In addition, control of exposure times, which must necessarily be short to avoid the occurrence of reciprocity failure in the film, becomes extremely critical as the frequency of the drive signal is reduced. For example, for a half second exposure at 60 Hz., only 30 voltage cycles will be applied to the lamp, and an error of only .+-.1 cycle will result in approximately a .+-.3% error in the light produced by the lamp. Thus, it will be appreciated that the potential for reciprocity failure or inconsistent exposure is increased with such prior art units which use drive frequencies as low as 20 Hz.
The present invention seeks to avoid these problems by providing an improved dual color sensitometer having an electroluminescent lamp that is capable of selectively producing either of two distinct colors upon excitation from a single relatively high frequency AC signal. In particular, the electroluminescent lamp according to the present invention includes a plurality of phosphor elements in the shape of elongated strips. The elements are electrically divided into two sets, one for uniformly illuminating the surface with a first color (e.g., blue) and the other for illuminating the surface with a second color (e.g., green). In the preferred embodiment, all of the electroluminescent phosphor elements produce a single blue color upon excitation. However, one set of elements is covered with a second nonelectroluminescent phosphor adapted to emit light of a longer wavelength than the excitation color, preferably in the green color range. In addition, to insure a uniformity of light along the entire length of the graduated step tablet, the widths of the elongated phosphor elements vary in relation to their distance from the step tablet according to the inverse square law so that the elements farthest from the step tablet emit proportionately greater amounts of light.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :